Conventionally, digital transmission systems have been built to accommodate mainly telephone lines. SDH (Synchronous Digital Hierarchy), which is standardized by ITU-T, and SONET (Synchronous Optical Network), which is substantially the same standard as the SHD and is based on a U.S. ANSI standard, have been used as a digital hierarchy for transferring signals flowing through telephone lines efficiently, (cf. Non-patent Document 1).
Currently, data traffic has being increasing because of the spread of the Internet and so forth, in addition to the conventional voice traffic, and the data traffic has come to account for a large portion of the traffic. A variety of signals having various bit rates and formats have been standardized and used as the client signals accommodated in the network. For example, various client signals exist in the vicinity of 10 Gbit/s, such as 10 GbE LAN PHY (10.3125 Gbit/s), ODU2 (10.0373 Gbit/s), and STM-64 (9.95328 Gbit/s).
In view of such circumstances, the Optical Transport Network (OTN) (see Non-patent Document 2), which is predicated on wavelength division multiplexing transmission (WDM) system adaptable to an explosive increase of the Internet traffic, is standardized by the ITU-T as a platform for transparently transmitting various client signals such as ATM and Ethernet (registered trademark), not just SONET/SDH. For example, introduction of the network utilizing an OTN 1201 such as shown in FIG. 12 has been in progress rapidly.
FIG. 12 is a configuration diagram showing one example of a conventional network configuration. FIG. 12 shows that the OTN 1201 is connected to a SONET/SDH network 1202 and an Ethernet network 1203. More specifically, the OTN 1201 is connected to the SONET/SDH network 1202 via a transponder 1205 for SONET/SDH, which is provided in a transmission device 1204, and the OTN 1201 is also connected to the Ethernet network 1203 via a transponder 1207 for Ethernet, which is provided in a transmission device 1206.
In addition, user's need for transparent transmission, i.e., transmitting data in the original data form to a receiver in data communication, has been increasing in recent years. For example, the need for transparent transmission has been demonstrated by the fact that a scheme for transparently accommodating 10 GbE LAN PHY signal in the OTN platform (overclocked OTU2) has been discussed in the ITU-T and documented as G.Sup43 (see Non-patent Document 3).
Moreover, mechanisms for performing rate adjustment of a plurality of client signals are shown in GFP (Generic framing procedure) (see Non-patent Document 4), which is specified in ITU-T Recommendation G.7041, and Japanese Patent No. 3480444 (see Patent Document 1), which is an extension thereof. However, these have problems such that both of them are predicated on a specific encoding method (8B/10B encoding) and that they are incapable of transparent transmission.
Furthermore, there have been increasing demands for connecting LAN (Local Area Network) environments existing in remote locations directly with LAY-PHY (physical layer). In addition, there are users and device vendors who use the preamble and the inter frame gap (IFG) in the Ethernet signal with customized specifications that are different from the IEEE standard (see Non-patent Document 5). Therefore, carrier network is required to transparently transfer the entire signal including the preamble and the IFG, in addition to the frame, for some Ethernet signals whose bit rates are not multiples of integral.
However, the client signal of the OTN is predicated on the SONET/SDH signal. The bit rate of the client signal of the OTN is defined as 2.48832 Gbit/s, 9.95328 Gbit/s, and 39.81312 Gbit/s, which are different from the bit rates of the 1 GbE signal and the 10 GbE signal. An OTN to which the above-described overclocking scheme is applied is widely used as a method for transparently and efficiently accommodating 10 GbE LAN PHY signal, which has a different bit rate from the bit rate of the currently standardized OTN client signal. The application of the overclocking scheme refers to accommodating the 10 GbE LAN PHY signal with 10.3125 Gbit/s as it is as a client signal in the payload of the OTN without making any change to the frame structure and function of the OTN by increasing of the bit rate alone at a rate of 10.3125/9.95328.
FIG. 13 shows a configuration diagram of a digital transmission system, according to a conventional technique, in which a STM-64 signal and a 10 GbE LAN PHY signal are accommodated directly in a payload area of an OTU frame and transmitted by wavelength division multiplexing, and a configuration diagram of an OTU frame structure. When accommodating a 10 GbE LAN PHY signal 1302a in an OTU frame shown in the lower portion of FIG. 13, the overclocking scheme is applied in a transmission device 1304, and the bit rate is increased by adding an overhead, FS (Fixed Stuff) byte, and FEC, so that the 10 GbE LAN PHY signal is accommodated directly in the OTU frame with 11.0957 Gbit/s. The OTN is predicated on WDM, so a large capacity system can be realized by wavelength division multiplexing even if the bit rates differ at different wavelengths. For this reason, the OTN is used widely. Moreover, additional signal processing is unnecessary even when accommodating 10 GbE-LAN PHY signal, and efficient accommodating is possible at low cost. Therefore, this technique is documented in the ITU-T, as described above (see Non-patent Document 3).
As described referring to FIG. 12, when accommodating various client signals in the conventional network, respective transponders are prepared for respective client signals (the transponder 1205 for SONET/SDH and the transponder 1207 for Ethernet) to accommodate the client signals. Also, when accommodating the 10 GbE LAN PHY signal in the OTU frame using the overclocking scheme, the bit rate is different from the case where a STM-64 signal is accommodated, so a transmission signal having a plurality of bit rates need to be subjected to wavelength division multiplexing. However, preparing transponders (optical transmission-reception modules or frame processing circuits) corresponding to the types of client signals leads to the problems of lack of flexibility in transponder arrangement and also high cost.
In addition, in the conventional networks, rate adjustment was performed by decoding the encoded client signal and reducing the bit rate when handling a variety of both SDH-based and Ethernet-based client signals, for the reasons of, for example, the difference in bit rate between the client signal and the payload of the network which accommodates the signal. For example, a client signal using the 8B/10B encoding method is decoded so that the bit rate is reduced to 80%. Further, part of the client signal is deleted so that the rate is adjusted. For example, IFG is deleted when transferring a 10 GbE LAN PHY signal so that the bit rate is reduced. However, such methods in which the transparency is reduced have the problem that they cannot meet the recently increasing user's need for transparent transmission.
In addition, when multiplexing a client signal by a 40 Gbit/s OTN system, the difference in the bit rate cannot be permitted because a 10 Gbit/s signal is time-division multiplexed to a 40 Gbit/s signal. That is, although a normal 40 Gbit/s OTN system can multiplex-accommodate a STM-64 signal, it cannot multiplex-accommodate a 10 GbE LAN PHY signal, which has a different bit rate. In addition, although an overclocked 40 Gbit/s OTN system can multiplex-accommodate the 10 GbE LAN PHY signal, it cannot multiplex-accommodate the STM-64 signal. Therefore, a STM-64 signal and a 10 GbE LAN PHY signal cannot be allowed to coexist and multiplexed in a 40 Gbit/s signal with one wavelength. Therefore, there has been a problem that, in the case of transmitting a STM-64 signal and a 10 GbE LAN PHY signal separately, 2 wavelengths of 40 Gbit/s signals are necessary for wavelength division multiplexing of 40 Gbit/s in which STM-64 signals are multiplexed and 40 Gbit/s in which 10 GbE LAN PHY signals are multiplexed, so the trunking efficiency cannot be increased.
The invention has been accomplished in view of such problems. It is an object of the invention to provide a digital transmission system and a digital transmission method that are capable of accommodating, or accommodating and multiplexing, various client signals having different bit rates and that realize transparent transfer of various client signals at low cost and improved trunking efficiency.    [Patent Document 1] Japanese Patent No. 3,480,444    [Non-patent Document 1] ITU-T G.707, “Network node interface for the synchronous digital hierarchy (SDH)”    [Non-patent Document 2] ITU-T G.709, “Interfaces for the Optical Transport Network (OTN)”    [Non-patent Document 3] ITU-T G.Sup43, “Transport of IEEE 10G Base-R in Optical Transport Networks (OTN)”    [Non-patent Document 4] ITU-T G.7041, “Generic framing procedure (GFP)”    [Non-patent Document 5]H. Ichino et al., IJHSES, vol. 15, no. 3, pp. 191-228, 2005